python图像处理裁剪,OpenCV图像处理以在Python中裁剪图像的倾斜部分

I am trying to crop a portion of an image as shown below using opencv / PIL . I want to crop the rectangle area as shown in red lines in the image in the below link. It is tilted at an angle.

I used numpy slicing logic as below. But it doesn't crop at an angle. It crops a normal straight rectangle

rect = cv2.boundingRect(pts)

x,y,w,h = rect

cropped = img[y:y+h, x:x+w]

Also tried rotating the entire image at an angle and then cropping that part but it shrinks the resulting image

I am able to draw a rectangle on that image using the below code :

def draw_angled_rec(x0, y0, width, height, angle, img):

_angle = angle * math.pi / 180.0

b = math.cos(_angle) * 0.5

a = math.sin(_angle) * 0.5

pt0 = (int(x0 - a * height - b * width),

int(y0 + b * height - a * width))

pt1 = (int(x0 + a * height - b * width),

int(y0 - b * height - a * width))

pt2 = (int(2 * x0 - pt0[0]), int(2 * y0 - pt0[1]))

pt3 = (int(2 * x0 - pt1[0]), int(2 * y0 - pt1[1]))

cv2.line(img, pt0, pt1, (255,0,0), 3)

cv2.line(img, pt1, pt2, (255,0,0), 3)

cv2.line(img, pt2, pt3, (255,0,0), 3)

cv2.line(img, pt3, pt0, (255,0,0), 3)

Please suggest / advice a way to achieve it.

Thanks

解决方案

Here's a image extraction widget that allows you to rotate the image and select a ROI by clicking and dragging the mouse. The idea is to use the mouse to select the bounding box window where we can use Numpy slicing to crop the image. Since OpenCV does not let you draw an angled rectangle, you can bypass that by first rotating the image.

Once you have selected the ROI, you can then crop the image using the bounding box coordinates. If we consider (0,0) as the top left corner of the image with left-to-right as the x-direction and top-to-bottom as the y-direction and we have (x1, y1) as the top-left vertex and (x2,y2) as the bottom-right vertex of a ROI, we can crop the image by:

ROI = image[y1:y2, x1:x2]

We are able to do this since images are stored as a Numpy array in OpenCV. Here is a great resource for Numpy array indexing and slicing.

To use the widget:

left mouse click + drag - select ROI

right mouse click - reset image

r - rotate image clockwise 5 degrees

e - rotate image counter-clockwise 5 degrees

c - crop selected ROI

q - quit program

import cv2

import numpy as np

class ExtractImageWidget(object):

def __init__(self):

self.original_image = cv2.imread('plane.PNG')

# Resize image, remove if you want raw image size

self.original_image = cv2.resize(self.original_image, (640, 556))

self.clone = self.original_image.copy()

cv2.namedWindow('image')

cv2.setMouseCallback('image', self.extract_coordinates)

# Bounding box reference points and boolean if we are extracting coordinates

self.image_coordinates = []

self.angle = 0

self.extract = False

self.selected_ROI = False

def extract_coordinates(self, event, x, y, flags, parameters):

# Record starting (x,y) coordinates on left mouse button click

if event == cv2.EVENT_LBUTTONDOWN:

self.image_coordinates = [(x,y)]

self.extract = True

# Record ending (x,y) coordintes on left mouse bottom release

elif event == cv2.EVENT_LBUTTONUP:

self.image_coordinates.append((x,y))

self.extract = False

self.selected_ROI = True

self.crop_ROI()

# Draw rectangle around ROI

cv2.rectangle(self.clone, self.image_coordinates[0], self.image_coordinates[1], (0,255,0), 2)

cv2.imshow("image", self.clone)

# Clear drawing boxes on right mouse button click and reset angle

elif event == cv2.EVENT_RBUTTONDOWN:

self.clone = self.original_image.copy()

self.angle = 0

self.selected_ROI = False

def show_image(self):

return self.clone

def rotate_image(self, angle):

# Grab the dimensions of the image and then determine the center

(h, w) = self.original_image.shape[:2]

(cX, cY) = (w / 2, h / 2)

self.angle += angle

# grab the rotation matrix (applying the negative of the

# angle to rotate clockwise), then grab the sine and cosine

# (i.e., the rotation components of the matrix)

M = cv2.getRotationMatrix2D((cX, cY), -self.angle, 1.0)

cos = np.abs(M[0, 0])

sin = np.abs(M[0, 1])

# Compute the new bounding dimensions of the image

nW = int((h * sin) + (w * cos))

nH = int((h * cos) + (w * sin))

# Adjust the rotation matrix to take into account translation

M[0, 2] += (nW / 2) - cX

M[1, 2] += (nH / 2) - cY

# Perform the actual rotation and return the image

self.clone = cv2.warpAffine(self.original_image, M, (nW, nH))

self.selected_ROI = False

def crop_ROI(self):

if self.selected_ROI:

self.cropped_image = self.clone.copy()

x1 = self.image_coordinates[0][0]

y1 = self.image_coordinates[0][1]

x2 = self.image_coordinates[1][0]

y2 = self.image_coordinates[1][1]

self.cropped_image = self.cropped_image[y1:y2, x1:x2]

print('Cropped image: {} {}'.format(self.image_coordinates[0], self.image_coordinates[1]))

else:

print('Select ROI to crop before cropping')

def show_cropped_ROI(self):

cv2.imshow('cropped image', self.cropped_image)

if __name__ == '__main__':

extract_image_widget = ExtractImageWidget()

while True:

cv2.imshow('image', extract_image_widget.show_image())

key = cv2.waitKey(1)

# Rotate clockwise 5 degrees

if key == ord('r'):

extract_image_widget.rotate_image(5)

# Rotate counter clockwise 5 degrees

if key == ord('e'):

extract_image_widget.rotate_image(-5)

# Close program with keyboard 'q'

if key == ord('q'):

cv2.destroyAllWindows()

exit(1)

# Crop image

if key == ord('c'):

extract_image_widget.show_cropped_ROI()

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